Implantable lead functional status monitor and method

ABSTRACT

A system for monitoring trends in lead impedance includes collecting data from various sources in an implantable medical device system. Lead impedance, non-physiologic sensed events percentage of time in mode switch, results of capture management operation, sensed events, adversion pace counts, refractory sense counts and similar data are used to determine the status of a lead. A set of weighted sum rules are implemented by a software system to process the data and provide displayable information to health care professionals via a programmer. The lead monitoring system includes a patient alert system for patients to seek help in the event a serious lead condition is identified.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 09/487,562filed on Jan. 19, 2000 now U.S. Pat. No. 6,317,633 that is incorporatedby reference herein in its entirety.

FIELD OF THE INVENTION

This invention relates generally to leads used with implantable medicaldevices. Specifically, it relates to the monitoring of a lead'sfunctional status, the storage of lead-related data, and aninterpretation of these data into a report for use by the clinician.

BACKGROUND OF THE INVENTION

A wide assortment of automatic, body-implantable medical devices (IMDs)are presently known and commercially available. The class of suchdevices includes cardiac pacemakers, cardiac defibrillators andcardioverters, neural stimulators, among others. The leads used in theseIMDs extend from the device through a plurality of pathways into oradjacent to various chambers of the heart, deep into the brain, into alocation within the spine, and into or adjacent to other body organs,muscles and nerves, among others.

Many state-of-the-art pacemakers are capable of performing eitherunipolar or bipolar sensing and pacing in chambers of the heart.Unipolar pacing requires a lead with one insulated conductor and onedistal pacing electrode disposed thereon. As will be appreciated bythose of ordinary skill in the art, in most unipolar configurations, thecasing of the implantable pulse generator (IPG) is conductive andfunctions as the indifferent electrode in pacing or sensing. Bipolarpacing and/or sensing, on the other hand, uses a lead with two mutuallyisolated conductors and two electrodes located in the heart. Typically,one electrode is disposed at the distal end of the lead and is referredto as the “tip” electrode, while the second electrode is locatedsomewhat proximally from the tip electrode and is referred to as a“ring” electrode.

Generally, the leads are constructed of small diameter, highly flexible,reliable lead bodies made to withstand degradation by body fluids. Inaddition, they must be able to function in the presence of dynamic bodyenvironments that apply stress and strain to the lead body and theconnections made to electrodes or sensor terminals. Some of thesestresses may occur during the implantation process. Months or yearslater, porosity that developed from those stresses may be magnified byexposure to body fluids. These, in turn, may result in conductor orinsulation related conditions that may be manifested in an intermittentor sudden Loss of Capture (LOC), out-of-range impedance and/or Loss ofSensing (LOS).

Many state-of-the-art pacemakers can be programmed to operate in eitherunipolar or bipolar pacing and sensing configurations using implantedleads that are responsive to changes in the patient's therapy needs.This gives the implanting physician considerable flexibility inconfiguring a pacing system to suit the particular needs of a patient.The state of the art in current use of leads is not completely failsafe. For example, one of the two conductors or electrodes on animplanted bipolar lead may fail for various reason, for example, a leadmay fail because of breakage of a conductor due to metal fatigue, poorconnection(s) between the lead(s) and the pacemaker itself, subclaviancrushing of the lead, metal ion oxidation and a short circuit due tourethane/silicone breakdown. In such cases, it would be necessary tore-program the lead configuration manually or automatically to unipolarpacing and sensing in order for the pacemaker to function properly.Under current medical practice, the need for re-programming only becomesapparent upon careful examination of the patient in a clinical setting.These follow-up sessions, however, may not be conducted frequentlyenough to ensure proper operation of the pacemaker between suchsessions.

Other problems may arise at the proximal lead end that is placed intothe lead connector assembly and electrically connected via a “screw” orother connective means during implant. Due to improper connection duringimplant, the pacing signal may become intermittently or continuouslydisrupted, resulting in a high impedance or open circuit. Alternatively,the lead's distal end may become dislodged from cardiac tissue,resulting in intermittent or continuous LOC in one or both chambers.“Lead penetration” may occur during implantation when the distal end ofthe lead is advanced too far and protrudes through the myocardium. “Exitblock”, though rare, may occur due to inflammation of the cardiac tissuein contact with the distal electrode surface. The inflammation reachessuch a level that either total LOC and/or LOS occurs.

When these lead problems manifest themselves, it is necessary for theclinician to diagnose the nature of the lead related condition from theavailable data, IMD test routines, and patient symptoms. Once diagnosed,the clinician must take corrective action, for example, reprogram tounipolar polarity, open the pocket to replace the lead, reposition theelectrodes or sensors, or tighten the proximal connection.

Certain IMDs, that have been clinically used or proposed, rely onlead-borne physiologic sensors that monitor physiologic conditions, forexample, without limitation, blood pressure, temperature, pH, and bloodgases. The operation of these sensors also depends on the integrity ofthe leads to which they are connected.

Lead impedance data and other parameter data, for example, withoutlimitation, battery voltage, switching from bipolar to unipolarconfiguration, error counts, and LOC/LOS data, may be compiled anddisplayed on a programmer screen and/or printed out for analysis by theclinician. The clinician may also undertake real time IMD parameterreprogramming and testing while observing the monitored surface ECG totry to pinpoint a suspected lead related condition that is indicated bythe data and/or patient and/or device symptoms.

Several approaches have been suggested to provide physicians withinformation and/or early detection or prevention of these lead-relatedconditions. Commonly assigned U.S. Pat. No. 5,861,012 (Stroebel),incorporated herein by reference, describes several approaches toautomatically determine the pacing threshold. Periodically, a pacingthreshold test is conducted wherein the pacing pulse width and amplitudeare reduced to determine chronaxie and rheobase values to capture theheart. These threshold test data are stored in memory, and used tocalculate a “safety margin” to ensure capture.

Certain external programmers that address the analysis of such data andsymptoms include those disclosed in the following U.S. Pat. Nos.:4,825,869 (Sasmor et al.); 5,660,183 (Chiang et al.); and 5,891,179 (ERet al.), all incorporated herein by reference. The '869 patent describesprocessing a variety of uplinked, telemetered atrial and ventricular EGMdata, stored parameter and event data, and the surface ECG in rule-basedalgorithms for determining various IPG and lead malfunctions. The '183patent also considers patient symptoms in an interactive probabilitybased expert system that compares data and patient systems to storeddiagnostic rules, relating symptoms to etiologies so as to develop aprognosis. The '179 patent discloses a programmer that can be operatedto provide a kind of time-varying display of lead impedance values inrelation to upper and lower impedance limits. The lead impedance valuesare derived from pacing output pulse current and voltage values. Thesevalues are then either measured and stored in the IPG memory from anearlier time or represent current, real-time values that are telemeteredto the programmer for processing and display.

Prior art detection of lead-related conditions and various IPG responsesto such detection are set forth in the following U.S. Pat. Nos.:4,140,131 (Dutcher et al.); 4,549,548 (Wittkampf et al.); 4,606,349(Livingston et al.); 4,899,750 (Ekwall); 5,003,975 (Hafelfinger et al.);5,137,021 (Wayne et al.); 5,156,149 (Hudrlik); 5,184,614 (Collins);5,201,808 (Steinhaus et al.); 5,201,865 (Kuehn); 5,224,475 (Berg etal.); 5,344,430 (Berg et al.); 5,350,410 (Kieks et al.); 5,431,692(Hansen et al.); 5,453,468 (Williams et al.); 5,507,786 (Morgan et al.);5,534,018 (Walhstrand et al.); 5,549,646 (Katz et al.); 5,722,997(Nedungadi et al.); 5,741,311 (McVenes et al.); 5,755,742 (Schuelke etal.); and 5,814,088 (Paul et al.). All of these patents are incorporatedherein by reference.

Most of the above-cited patents generally disclose systems forperiodically measuring lead impedance and comparing the impedancemeasurements with upper and lower values or ranges and either storingthe data for later retrieval, and/or changing a pacing orcardioversion/defibrillation path, and/or adjusting the delivered pacingenergy, and/or alerting the patient by generating sound or stimulationwarning signals.

The aforementioned P-8050 filed invention discloses a lead statusmonitor (LSM) incorporated into an IMD for processing lead-related datainto a system self-test mode. The LSM provides a lead status report thatidentifies and declares conductor/connector issues, insulation issues,and electrode/tissue interface issues indicative of suspected leadrelated condition mechanisms for each lead employed in the IMD. The LSMoperates employing a set of rules that process measured lead impedancevalues and changes in pacing pulse amplitude required to capture.

Accordingly, there is a need for a self-testing system to provide a leadstatus report that identifies particular lead related condition(s) foreach lead employed in the IMD based on other measured values in additionto lead impedances and/or counts of various event types. What is neededis an LSM that would include measurement methods and techniques otherthan impedance or event counts. Such an LSM would preferably provide arule-based algorithm for determining various lead malfunctions, a leadstatus report, a patient alert, and messages to clinicians to alter theIMD operating mode and/or to discontinue using a defective lead.

SUMMARY OF THE INVENTION

The present invention includes an LSM incorporated into an IMD forprocessing lead related data and providing a lead status report. The LSMderives its data from various sources including, for example, leadimpedance, non-physiologic sensed (NPS) events, percentage of time inmode switch, results of capture management operation, sensed events,reversion pace counts, refractory sense counts. Data from these sourcesidentifies lead conductor/connector issues, lead insulation issues, andelectrode/tissue interface issues indicative of lead-related mechanismssuggestive of impending or actual lead failure for each lead employed inthe IMD. The LSM employs a set of weighted sum rules used by algorithmsto process data from all the above-mentioned sources to arrive at easilyinterpreted messages accessible to clinicians via the externalprogrammer.

The LSM report displayed by the external programmer provides theclinician with a clear interpretation of stored lead-related data. Afterreading this report, the clinician is able to determine the lead statusmuch faster than by using tests to verify lead data that have beentraditionally presented by the programmer. The interpretation of the rawdata along with suggestions as to further action will simplify andshorten follow-up procedures.

Optionally, the LSM may include a patient-alert capability that notifiesthe patient to seek assistance whenever a serious lead-related conditionis identified or indicated using the LSM weighted sum rules.Additionally, the LSM includes the ability to switch the polarityconfiguration to one that provides an integral pathway for pacing andsensing.

Economic and operational efficiency requires that the number of periodicfollow-up sessions to assess the patient's condition and the integrityof the IMD lead system be limited without comprising patient safety andcare. With the assistance of the LSM and its associated messages andreports, the number of follow-up sessions may be reduced in numberand/or conducted by medical personnel other than physicians therebyreducing costs. The present invention is expected to increase confidencein the self-monitoring capability of the IMD.

These and other advantages and features of the present invention will bemore readily understood from the following detailed descriptions anddrawings of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a body-implantable device system inaccordance with the present invention, including a hermetically sealeddevice implanted in a patient and an external programming unit.

FIG. 2 is a perspective view of the external programming unit of FIG. 1.

FIG. 3 is a block diagram of the implanted device from FIG. 1.

FIG. 4 is a flow diagram depicting the various factors used by the LSMalgorithm and the general types of messages issued by the algorithm.

FIG. 5 is an illustration of an embodiment implemented to determineshort and open circuit impedances from a typical output impedancemeasurement.

FIG. 6 is an illustration of a preferred alternative embodiment used toassess lead impedances via a subthreshold pulse.

FIG. 7a, b, and c illustrate the difference between nativedepolarization waves and non-physiologic sensed events, and how theseevents can then be counted to help determine a lead-related condition.

FIG. 8 is a table that relates the count of non-physiologic sensedevents to the weighted sum algorithm.

FIGS. 9a and b are illustrations depicting how the automatic sensingassurance method may help determine a lead-related condition.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an implantable medical device systemadapted for use in accordance with the present invention. The medicaldevice system shown in FIG. 1 includes an implantable device 10, forexample, a pacemaker that has been implanted in patient 12. Inaccordance with conventional practice in the art, pacemaker 10 is housedwithin a hermetically sealed, biologically inert outer casing, which mayitself be conductive so as to serve as an indifferent electrode in thepacemaker's pacing/sensing circuit. One or more pacemaker leads,collectively identified with reference numeral 14 in FIG. 1 areelectrically coupled to pacemaker 10 in a conventional manner and extendinto the patient's heart 16 via a vein 18. Disposed generally near thedistal end of leads 14 are one or more exposed conductive electrodes forreceiving electrical cardiac signals and/or for delivering electricalpacing stimuli to heart 16. As will be appreciated by those of ordinaryskill in the art, leads 14 may be implanted with their distal end(s)situated in the atrium and/or ventricle of heart 16.

Although the present invention will be described herein with referenceto an embodiment that includes a pacemaker, those of ordinary skill inthe art having the benefit of the present disclosure will appreciatethat the present invention may be advantageously practiced in connectionwith numerous other types of IMD systems, and indeed in any applicationin which it is desirable to provide a method to determine measurementsand identification of other quantifiable data available from implantedlead systems.

Also depicted in FIG. 1 is an external programming unit 20 fornon-invasive communication with implanted device 10 via uplink anddownlink communication channels, to be hereinafter described in furtherdetail. Associated with programming unit 20 is a programming head 22, inaccordance with conventional medical device programming systems, forfacilitating two-way communication between implanted device 10 andprogrammer 20. In many known implantable device systems, a programminghead such as that depicted in FIG. 1 is positioned on the patient's bodyover the implant site of the device, generally, within 2- to 6-inches ofskin contact depending on telemetry type used, such that one or moreantennae within the head can send RF signals to, and receive RF signalsfrom, an antenna disposed within the hermetic enclosure of the implanteddevice or disposed within the connector block of the device, inaccordance with known practice in the art.

FIG. 2 is a perspective view of programming unit 20 in accordance withthe presently disclosed invention. Internally, programmer 20 includes aprocessing unit (not shown in this Figure) that in accordance with thepresent invention is a personal computer type motherboard, e.g., acomputer motherboard including an Intel Pentium 3 or latermicroprocessor and related circuitry such as digital memory. The detailsof design and operation of the programmer's computer system will not beset forth in all its elements in the present disclosure, as it isbelieved that such details are well-known to those of ordinary skill inthe art.

Referring to FIG. 2, programmer 20 comprises an outer housing 60, thatis preferably made of thermal plastic or another suitably rugged yetrelatively lightweight material. A carrying handle, designated generallyas 62 in FIG. 2, is integrally formed into the front of housing 60. Withhandle 62, programmer 20 can be carried like a briefcase.

An articulating display screen 64 is disposed on the upper surface ofhousing 60. Display screen 64 folds down into a closed position (notshown) when programmer 20 is not in use, thereby reducing the size ofprogrammer 20 and protecting the display surface of display 64 duringtransportation and storage thereof.

A floppy disk drive is disposed within housing 60 and is accessible viaa disk insertion slot (not shown). A hard disk drive is also disposedwithin housing 60, and a hard disk drive activity indicator, (e.g., anLED, not shown) is provided to give a visible indication of hard diskactivation.

As would be appreciated by those of ordinary skill in the art, it isoften desirable to provide a means for determining the status of thepatient's conduction system. Programmer 20 is equipped with externalconnectors 24 that may be used to connect to pads placed on thepatient's body to detect ECG tracings from the implanted ECG leads.

In accordance with the present invention, programmer 20 is equipped withan internal printer (not shown) so that a hard copy of a patient's ECG,graphics, and reports displayed on the programmer's display screen 64can be generated. Several types of printers, such as the AR-100 printeravailable from General Scanning Co., are known and commerciallyavailable.

In the perspective view of FIG. 2, programmer 20 is shown witharticulating display screen 64 having been lifted up into one of aplurality of possible open positions such that the display area thereofis visible to a user situated in front of programmer 20. Articulatingdisplay screen is preferably of the LCD or electro-luminescent type,characterized by being relatively thin as compared, for example, acathode ray tube (CRT) or the like.

As would be appreciated by those of ordinary skill in the art, displayscreen 64 is operatively coupled to the computer circuitry disposedwithin housing 60 and is adapted to provide a visual display of graphicsand/or data under control of the internal computer.

Programmer 20 described herein with reference to FIG. 2 is described inmore detail in U.S. Pat. No. 5,345,362 issued to Thomas J. Winkler,entitled Portable Computer Apparatus With Articulating Display Panel,which patent is hereby incorporated herein by reference in its entirety.The Medtronic Model 9790 programmer, inter alia, is the implantabledevice-programming unit with which the present invention may be used todetermine the efficacy of the algorithms to be described below.

FIG. 3 is a block diagram of the electronic circuitry that typifiespulse generator 10 in accordance with the presently disclosed invention.As can be seen from FIG. 3, pacemaker 10 comprises a primary stimulationcontrol circuit 21 for controlling the device's pacing and sensingfunctions. The circuitry associated with stimulation control circuit 21may be of conventional design, in accordance, for example, with what isdisclosed in U.S. Pat. No. 5,052,388 issued to Sivula et al., Method AndApparatus For Implementing Activity Sensing In A Pulse Generator. To theextent that certain components of pulse generator 10 are conventional intheir design and operation, such components will not be described hereinin detail, as it is believed that design and implementation of suchcomponents would be a matter of routine to those of ordinary skill inthe art. For example, stimulation control circuit 21 in FIG. 3 includessense amplifier circuitry 25, stimulating pulse output circuitry 26, acrystal clock 28, a random-access memory and read-only memory (RAM/ROM)unit 30, and a central processing unit (CPU) 32, all of which arewell-known in the art.

Pacemaker 10 also includes internal communication circuit 34 so that itis capable of communicating with external programmer/control unit 20, asdescribed in FIG. 2 in greater detail.

With continued reference to FIG. 3, pulse generator 10 is coupled to oneor more leads 14 which, when implanted, extend transvenously between theimplant site of pulse generator 10 and the patient's heart 16, aspreviously noted with reference to FIG. 1. Physically, the connectionsbetween leads 14 and the various internal components of pulse generator10 are facilitated by means of a conventional connector block assembly11, shown in FIG. 1. Electrically, the coupling of the conductors ofleads and internal electrical components of pulse generator 10 may befacilitated by means of a lead interface circuit 19 which functions, ina multiplexer-like manner, to selectively and dynamically establishnecessary connections between various conductors in leads 14, including,for example, atrial tip and ring electrode conductors ATIP and ARING andventricular tip and ring electrode conductors VTIP and VRING, andindividual electrical components of pulse generator 10, as would befamiliar to those of ordinary skill in the art. For the sake of clarity,the specific connections between leads 14 and the various components ofpulse generator 10 are not shown in FIG. 3, although it will be clear tothose of ordinary skill in the art that, for example, leads 14 willnecessarily be coupled, either directly or indirectly, to senseamplifier circuitry 25 and stimulating pulse output circuit 26, inaccordance with common practice, such that cardiac electrical signalsmay be conveyed to sensing circuitry 25, and such that stimulatingpulses may be delivered to cardiac tissue, via leads 14. Also not shownin FIG. 3 is the protection circuitry commonly included in implanteddevices to protect, for example, the sensing circuitry of the devicefrom high voltage stimulating pulses.

As previously noted, stimulation control circuit 21 includes centralprocessing unit 32 which may be an off-the-shelf programmablemicroprocessor or micro controller, but in the present invention is acustom integrated circuit. Although specific connections between CPU 32and other components of stimulation control circuit 21 are not shown inFIG. 3, it will be apparent to those of ordinary skill in the art thatCPU 32 functions to control the timed operation of stimulating pulseoutput circuit 26 and sense amplifier circuit 25 under control ofprogramming stored in RAM/ROM unit 30. It is believed that those ofordinary skill in the art will be familiar with such an operativearrangement.

With continued reference to FIG. 3, crystal oscillator circuit 28 may bea 32,768-Hz crystal controlled oscillator that provides main timingclock signals to stimulation control circuit 21. Again, the lines overwhich such clocking signals are provided to the various timed componentsof pulse generator 10 (e.g., microprocessor 32) are omitted from FIG. 3for the sake of clarity.

It is to be understood that the various components of pulse generator 10depicted in FIG. 3 are powered by means of a battery (not shown) that iscontained within the hermetic enclosure of pacemaker 10, in accordancewith common practice in the art. For the sake of clarity in the Figures,the battery and the connections between it and the other components ofpulse generator 10 are not shown.

Stimulating pulse output circuit 26, which functions to generate cardiacstimuli under control of signals issued by CPU 32, may be, for example,of the type disclosed in U.S. Pat. No. 4,476,868 to Thompson, entitledBody Stimulator Output Circuit, which patent is hereby incorporated byreference herein in its entirety. Again, however, it is believed thatthose of ordinary skill in the art could select from among many varioustypes of prior art pacing output circuits that would be suitable for thepurposes of practicing the present invention.

Sense amplifier circuit 25, which is of conventional design, functionsto receive electrical cardiac signals from leads 14 and to process suchsignals to derive event signals reflecting the occurrence of specificcardiac electrical events, including atrial depolarizations (P-waves)and ventricular depolarizations (R-waves). Sense amplifier 25 providesthese event-indicating signals to CPU 32 for use in controlling thesynchronous stimulating operations of pulse generator 10 in accordancewith common practice in the art. In addition, these event-indicatingsignals may be communicated, via uplink transmission, to externalprogramming unit 20 for visual display to a physician or clinician.

Those of ordinary skill in the art will appreciate that pacemaker 10 mayinclude numerous other components and subsystems, for example, activitysensors and associated circuitry. The presence or absence of suchadditional components in pacemaker 10, however, is not believed to bepertinent to the present invention, which relates to the determinationof measurements of lead impedance, detection of capture and/or LOC, aswell as other operations previous mentioned via algorithms that may beloaded into components described previously in FIG. 3.

FIG. 4 is a flow diagram depicting the various data sources used by LSMalgorithm 88 and the general types of messages issued by LSM algorithm88. The LSM requires measured and other types of data to determine thestatus of the IMD lead(s), or lack thereof.

Lead Impedance Measurements 70 have been used for identifying problemswith lead wire and/or insulation. The present invention uses asub-threshold pacing pulse to make impedance measurements. The LSMalgorithm 88 takes into account time factors, such as, for example,changes in impedance within 90 days post implant vs those that occurafter 90 days post implant. Thus, lead maturation is taken into accountby LSM algorithm 88. In addition, the present invention includes amethod to identify the extent of the increase or decrease in impedancemeasurements, even though these changes may not exceed the usual limitsof change, such as the nominal >4000 or <200 ohms that may be programmedto other values based on exceptional conditions or lead properties.Included also is a means to compare unipolar and bipolar impedancemeasurements on the same lead. LSM algorithm 88 is further discussedwith reference to in FIGS. 5 and 6.

Non-Physiologic Sensed (NPS) events 72 have usually been associated withsensing of electrical noise in the unipolar configuration. If, however,the circuitry can identify when a sensed event occurs at the same time ahigh frequency signal occurs, generally associated with a lead wiremake/break situation, these signals may be used as data to identifypotential lead issues. This is particularly true if such signals aredetected when the lead is in the bipolar configuration when the patientis asleep since, the assumption being while sleeping, the patient isless likely to be exposed to electrical interference. LSM algorithm 88is further discussed with reference to FIGS. 7 and 8.

The percent of time in mode switch 74 may also be indicative of anatrial lead sensing issue. Mode switching from an atrial tracking to anon-atrial tracking mode in the presence of high atrial rates wasdisclosed in U.S. Pat. No. 4,363,325, issued to Roline, et al. andincorporated herein by reference in its entirety. For example, when theatrial sensed rate exceeds a programmed upper tracking rate, IPGS andPCDs operating in a dual-chamber tracking mode (DDDR or DDD) may beprogrammed to switch to a non-tracking mode (DDI/R, DVI/R, or VVI/R).Mode switching may occur in response to physiologic events, such asatrial flutter or fibrillation, or in response to Non-Physiologic Sensed(NPS) events, such as those due to make/break contacts that can occurdue to fractured lead wire(s) or loose connectors. The present inventionenables identification of a sudden increase in the percentage of time inmode switch and, at the same time, determine if this increase is due tosensing of physiologic or NPS events. Greater detail on the frequency ofNPS events and how they are used by the LSM algorithm 88 is providedherein with reference to FIGS. 7 and 8.

Capture Management threshold methods 76, such as described in U.S. Pat.No. 5,861,013 issued to Peck, et al., incorporated herein by referencein its entirety, are familiar to those skilled in the art. Inconjunction with such methods, the present invention establishes areference threshold to which later capture management thresholdmeasurements 76 can be compared. Increasing thresholds are often anindication of a lead-related condition that may relate to a failinglead. Such a reference threshold takes into account potential issueswith the implanted lead(s), as well as biological/physiological issues,such as edema at the lead tissue interface, myocardial infarction, druginteractions and similar other conditions.

The reference threshold used for comparison purposes for the first 90days post-implant period is calculated at the first pacing post-implantthreshold search. Any upward threshold change exceeding a 0.25 to 1.0 Vrange at 1.0 ms, compared to the reference, during the first 90 daypost-implant period will trigger a message per blocks 90, 92, and 94discussed herein below. The 90-day reference threshold uses a 7-dayaverage calculated about day 90. Any threshold change (up or down)exceeding a 0.25 to 1.75V range at 1.0 ms, compared to the reference,after the 90 day post-implant period will trigger a message per blocks90, 92, and 94 discussed herein below.

Sensing Assurance settings 78, as disclosed in U.S. Pat. No. 6,112,119issued to Schuelke, et al., incorporated herein by reference in itsentirety, are also familiar to those skilled in the art. These methodsallow the device to measure R and P-wave amplitudes and compare these tocurrently programmed sensitivity settings. Lead-related conditions maycause a reduction in R and P-wave amplitudes. Changes in amplitude,which are associated with changes in sensitivity settings, may then beused as indicators of impending or actual lead-related conditions.

Reversion Pace Counts 80 is a useful source of information to helpidentify whether an implanted lead(s) may be on the verge of failing.Typically, an implanted pacemaker will revert to an asynchronous mode(DOO, AOO, VOO) when it is in the presence of continuous electricalnoise. A pacemaker, however, may not always distinguish betweenelectrical noise and sensing of frequent make/break connections as occurwhen a fractured lead wire's ends tap against each other due to heart orbody movements. Sensing such events may cause reversion to anasynchronous pacing mode. LSM algorithm 88, in accordance with thepresent invention, tracks the incidence of reversions against previouslydetected reference counts, and is a useful indicator of oversensing.

Refractory Sense Counts 82 may indicate any number of event types, suchas the presence of high-rate atrial or ventricular events,non-physiologic sensed events, among others. When such events aredetected in that portion of the refractory period occurring after aninitial blanking period, they will be sensed. By comparing the number ofrefractory sensed events to stored EGMs from the same time period, aphysician can visually determine the source of such events. In theabsence of such a visual comparison that may only take place during afollow-up session, LSM algorithm 88 can identify the presence ofnon-physiologic sensed events 72 and compare these to signals fromsensed physiologic events. Then, non-physiologic refractory sense counts82 can compare the incidence of refractory sensed events against areference count. If the number of refractory sense counts has increased,these data will be included in LSM weighted sum algorithm 88.

High Rate Episode Counts 84 may also be an indicator of sensingirregularities due to a failing or failed lead(s). An “episode” is usedto describe a continuous high atrial or ventricular rate that extendsover a period of, at least, a second or two, as opposed to discreteevents. Events, constituting an episode, may be refractory ornon-refractory sensed events. By tracking the number of high rateepisodes from one follow-up session to another, an increase in theirincidence may be used as a potential indicator of sensingirregularities. Such changes may be included in LSM weighted sumalgorithm 88.

The Time from Implant 86, by default, is a 90-day timer that starts withthe implant of the IMD, that is, when the lead(s) is connected to theIMD. When an MD is “changed out,” due perhaps to the need for a batterychange, the IMD time from implant will be different from the lead(s)time from implant. A similar situation may occur when an atrial lead mayhave a different implant date than a ventricular lead. In either case,the user will have the option of selecting, via the programmer, thedesired implant date that will become the new basis for LSM weighted sumalgorithm 88.

Lead performance data are collected at a higher frequency during thefirst week post-implant, since there is a greater chance forlead-related complications during this time. These data are saved forsix months. The same higher frequency of data collection automaticallygoes into effect whenever a sufficient number of parameters go out ofrange. Or, the physician can program the higher frequency of detectiontool to assist in diagnosing lead-related conditions.

Weighted sum algorithm 88 uses individual weights assigned to itemslisted above (70 through 86). In addition, weighted sum algorithm 88stores EGM data for physician review whenever an ambulatory or in-officelead status event (70 through 84) occurs. The algorithm sums the weightsand “interprets” them for the user in the following manner:

Lead-related parameters are all within range or operating normally;

One or more of the lead parameters are out-of-range. Investigate leads.

A number of lead parameters are out-of-range and a safety problemexists.

Polarity has been switched from bipolar to unipolar configuration forpacing sensing, and EGM collection.

Messages to the User 90, 92, and 94 refer to three types of lead-relatedconditions. Lead Conductor/Connector Messages 90 include the following:

High impedance (>4000 ohms, 2× increase over reference, among others),

Increase in threshold(s) above preset or programmed limit,

NPS counts above preset or programmed limit, and

Reduction in R and P-wave amplitude below preset or programmed limit.

Lead Insulation Messages 92 include the following:

Low impedance (<200 ohms, ½ decrease under reference, among others),

Increase in threshold(s) above preset or programmed limit,

NPS counts above preset or programmed limit, and

Reduction in R and P-wave amplitude below preset or programmed limit.

Biological Interface Messages 94 include the following:

Myocardial Penetration/Perforation

Increase in threshold(s) above preset or programmed limit, and

Reduction in R and P-wave amplitude below preset or programmed limit.

Lead Dislodgment

Increase in threshold(s) above preset or programmed limit, and

Reduction in R and P-wave amplitude below preset or programmed limit.

Exit Block

Increase in threshold(s) above preset or programmed limit, and

Possible reduction in impedance.

FIG. 5 illustrates an embodiment implemented to determine short and opencircuit impedances from a typical output impedance measurement inaccordance with prior art practice. The “droop” of a pacing output pulsemay be used to determine varying impedances on a lead system.

Pacing pulse 100, includes programmed output voltage 102 and pulse width104. The droop, or drop in voltage from the leading to the trailing edgeof the output pulse may be used to determine the presence of a short oropen circuit. Short circuit droop 106 indicates a low impedance (<200ohms) and is a sign of a breech in the lead's insulation. Typical droop108 is found in output pulses that range from ˜500 to 4000 ohms. Opencircuit droop 110 indicates a high lead impedance, >4000 ohms. Highimpedance loads are indicative of a break in the lead wire or an openelectrical connection between the lead and IMD.

Another embodiment similar to that shown in FIG. 5 is disclosed in U.S.Pat. No. 5,741,311 issued to McVenes, et al. and incorporated herein byreference in its entirety. In the '311 patent, a short burst, 150 to 125ms in duration with a current level of some 30 micro amps, followingpacing pulses only is used to calculate lead impedances.

If these embodiments are used within the context of the presentinvention, the frequency of impedance measurements will vary. During thefirst 3 months, impedance measurements will occur at a higher frequencyso as to insure detection of lead-related conditions such asperforation, dislodgment, and connector issues, among others. Anyout-of-range impedance measurements due to these or other issues will bereported via the programmer upon interrogation of the IMD. Those skilledin the art would understand that a nominal range of 200 to 4000 ohms isused as a guide to judge lead integrity. These are the values that arein effect during the first 90 days. Along with minimum and maximumimpedance limits, relative changes in impedance are also used as anindication of a lead-related condition.

FIG. 6 is an illustration of an alternate embodiment implemented toassess lead impedances via a subthreshold pulse. A small current isapplied to the lead during the cardiac refractory period, after either apaced or spontaneous depolarization. A further requirement is that thesubthreshold pulse must take place in the respective sense amplifier'sblanking period. Each pathway is measured, atrial or ventricular in bothconfigurations, unipolar and bipolar. The resulting voltage is usedalong with the current to calculate the impedance (R=V/I). The relativepositioning 112 of subthreshold pulse 114 occurs after pacing outputpulse 100. Subthreshold pulse 114 may be biphasic as shown here.

The frequency of impedance measurements will vary if the embodimentillustrated in FIG. 6 is implemented in accordance with the presentinvention. Particularly, during the first 3 months, impedancemeasurements will occur at a higher frequency so as to insure detectionof lead-related complications such as perforation, dislodgment, andconnector issues, among others. Any out-of-range impedance measurementsdue to these or other issues will be reported via the programmer uponinterrogation of the IMD. The same impedance ranges disclosed inrelation to FIG. 5 will also be used.

The embodiment illustrated in FIG. 6 offers several benefits. Pacing isnot required, since impedance measurements can be achieved even when thepatient is in his or her own intrinsic rhythm. Prior art system such asthe embodiment in FIG. 5 have several drawbacks. For example, voltageoutput 102 and pulse width 104 as programmed may be as high as 5 V and 1ms, or more. Some patients will feel this high voltage output.Subthreshold pulses may be used to measure both the unipolar and bipolarpathways. Paced impedances can measure only the pathway being used topace the patient. This is especially important when the bipolar pathwayis being used for sensing and the unipolar pathway is used for pacing,or vice versa.

Comparing unipolar and bipolar impedances may also lead to earlydetection of an insulation failure. A drop of more than 50 ohms may bedetected on the bipolar pathway as compared to the unipolar. Such adifference in impedance may indicate a break in the bipolar insulation.Body fluids can then invade the bipolar coil and electrically short thelead system.

The present invention may use any of the three embodiments describedabove to track trends in lead impedances. Physicians have long usedtrend analysis techniques to assess whether or not a lead was slowlyfailing. In the case of trends, however, it is important to rule out“infant” failures. To ensure that the LSM is dealing with a mature lead,a lead maturation time of 90 days post lead implant was chosen. Trendanalysis is used from implant to 90 days with implant values used as areference. Trend analysis is also used from 90 days forward withreference values collected around or near 90 days post-implant. In thepresent invention, the LSM algorithm logs lead impedance measurementsand tracks any trends. When the lead impedance is within the nominal 200to 4000 ohm range, a 20% increase in impedance from the “stable”reference measurement will trigger a message related to leadconductor/connector issues 90. Similarly, a 50% decrease in impedancefrom the “stable” reference measurement will trigger a message relatedto lead insulation issues 92.

FIG. 7a, b, and c illustrate the difference between nativedepolarization waves and non-physiologic sensed events, and how theseevents can then be counted to help determine a lead-related condition.FIG.7a illustrates the differences between Non-Physiologic Sensed (NPS)events and intrinsic sensed events as viewed on an electrogram (EGM).Intrinsic sensed events 120 display depolarization waves that aremarkedly different than NPS events 122. All NPS events 122 are countedexcept NPS event 123, due to its proximity to the previous NPS event.NPS event 123 occurred during the blanking period following the sensingof previous NPS event 122. This conclusion is validated by the fact thatNPS count 124 is not incremented at either count 1 or count 3, thelatter registering only the first NPS event 122, because the remainingevents are blanked.

FIG. 7b depicts hypothetical NPS counts accumulated preferably over asix-hour period 126. NPS counts 124 are broken down on the x-axis asthese occur within one-hour periods 128. Further, the NPS event datafrom various time periods are accumulated into 24-hour period 128 asdepicted in FIG. 7c. Depending on the sensitivity threshold 130 of theLSM algorithm, NPS counts may or may not qualify for consideration.

NPS events 122 may be defined as transient, high frequency signals whosecharacteristics may be differentiated via sense amp filters from signalsgenerated by intrinsic depolarization waveforms 120. The close proximityof such high frequency signals 122 to intrinsic depolarization waveforms120 may indicate the presence of a make/break situation where theconducting wire or connector has an integrity issue. NPS events 122,however, could also be due to sensing of electromagnetic interference(EMI). EMI may have significant impact on the operation and function ofunipolar pacing systems. The present invention will test for thepresence of NPS events 122 in both the unipolar and bipolarconfiguration, which is less susceptible to external noise. Moreover, bydetermining if there were a number of time windows in which NPS counts122 exceeded 24-hour threshold 130, a lead-related condition could bereasonably anticipated. Further, NPS counts 122 occurring during thepatient's normal sleeping hours provide further corroboration of thepossibility of a lead-related condition.

FIG. 8 represents the process by which the LSM weighted sum algorithmuses the count of non-physiologic sensed events to determine thepotential presence of a lead-related condition. The LSM algorithmevaluates the consistency of NPS events within the previous 24 hours.Since NPS events may be due to EMI, it is important to identify anddifferentiate bursts of EMI and eliminate them from consideration. Toachieve this desired outcome, the LSM algorithm uses the propertiesshown in the table.

The number of counter windows per day 132 may be programmed from amaximum of 24 to a minimum of two per day, resulting in the window sizesof one to 12 hours 134. During each counter window, the number of NPSevents must exceed the nominal or programmed number, or trip point,which differs for unipolar sensing 136 and bipolar sensing 138. Eachtime the NPS event counts exceed the trip point in either 136 or 138,the number of “successes” is registered in 140. For example, in row 2with a programmed 24 windows per day, 16 of the 24 windows must haveregistered NPS counts above the programmed or nominal trip point. If thetrip point was reached in 16 or more windows, these data are stored forfurther review by the clinician. If not, new measurements will overwritethe existing data.

FIG. 9a depicts the relationship between the amplitude of a hypotheticalsensed endocardial signal and the programmed or automatically selectedsensitivity settings 142. Endocardial signal 144 is sensed by thepacemaker and displayed on the programmer screen as an electrogramtracing. The pacemaker's sense amplifier related this signal toamplitude 144 and uplinks this datum as reported amplitude 146, 5.6 mVin this hypothetical example. Applying the 2:1 safety margin rule, thisdatum is halved resulting in selected sensitivity setting 148 or 2.8 mVin this hypothetical example.

FIG. 9b illustrates the process by which the sensing assurancealgorithm, implemented in the Medtronic® Kappa® 700 IPG, utilizes thedata shown in FIG. 9a. The sensing assurance algorithm has senseamplitude classification 150 and evaluates all sensed events. Optimally,the amplitude of the sensed signals will be detected within Sense MarginIndicator (SMI) High 152 and SMI Low 154. When this occurs, the sensedcardiac signal is determined to be an adequate sense 160. If theamplitude of the sensed signal is above SMI high 152, it is declared tobe a high amplitude sense 158 and the SMI High count is increased byunity. If or when the high counts accumulate to a certain level,sensitivity threshold 156 is increased by one setting, for example, from2.8 to 4.0 mV. On the other hand, a low amplitude sense 162 is a sensedevent that falls below SMI Low 154. Further, if or when the low countsaccumulate to a certain level, sensitivity threshold 156 is decreased byone setting, for example, from 2.8 to 2.0 mV.

The Kappa 700 implementation of sensing assurance is an initialembodiment and, with finer increments in the sensitivity settings asenvisioned in the present invention, may be used to establish referenceP and R amplitudes. From these references, amplitude ranges may beestablished against which real time amplitude measurements may becompared to determine whether there is an indication of a lead-relatedcondition.

The preceding specific embodiments are illustrative of the practice ofthe invention. It is to be understood, therefore, that other expedientsknown to those of skill in the art or disclosed herein may be employedwithout departing from the invention or the scope of the appended claim.It is therefore to be understood that the invention may be practicedotherwise than is specifically described, without departing from thescope of the present invention. As to every element, it may be replacedby any one of infinite equivalent alternatives, only some of which aredisclosed in the specification.

What is claimed is:
 1. A lead status monitor system comprising: animplantable medical device; a lead electrically coupled to saidimplantable medical device; and a software implemented in the processorof said implantable medical device able to perform weighted sum analysisto display messages relating to said lead status.
 2. The system of claim1 wherein said weighted sum analysis is based on a lead impedancemeasurement.
 3. The system of claim 1 wherein a said weighted sum isbased on a non-physiologic sensed event.
 4. The system of claim 1wherein a said weighted sum is based on percent time mode switch.
 5. Thesystem of claim 1 wherein a said weighted sum is based on capturemanagement thresholds.
 6. The system of claim 1 wherein a said weightedsum is based on sensing assurance settings.
 7. The system of claim 1wherein a said weighted sum is based on reversion pace counts.
 8. Thesystem of claim 1 wherein a said weighted sum is based on refractorysense counts.
 9. The system of claim 1 wherein a said weighted sum isbased on high rate episode counts.
 10. The system of claim 1 wherein asaid weighted sum is based on time from implant.
 11. The system of claim1 wherein said messages relate to lead insulation issues.
 12. The systemof claim 1 wherein said messages relate to biological lead interfaceissues.
 13. The system of claim 1 wherein said messages relate to leadindicator/connector issues.
 14. The system of claim 13 wherein saidmessage relating to lead conductor/connector issues is based on a leadimpedance measurement of a 200% increase in impedance from a stablereference measurement.
 15. The system of claim 11 wherein said messagerelating to lead insulation issues is based on a lead impedancemeasurement of a 50% decrease in impedance from a stable referencemeasurement.